t division wall to be formed in the embryo is transverse,
and is followed by vertical ones (Fig. 54, _E_, _em._). As the
embryo enlarges, the walls of the basal part of the archegonium grow
rapidly, so that the embryo remains enclosed in the archegonium
until it is nearly full-grown (Fig. 55). As it increases in size, it
becomes differentiated into three parts: a wedge-shaped base or
"foot" penetrating downward into the upper part of the plant, and
serving to supply the embryo with nourishment; second, a stalk
supporting the third part, the capsule or spore-bearing portion of
the fruit. The capsule is further differentiated into a wall, which
later becomes dark colored, and a central cavity, in which are
developed special cells, some of which by further division into four
parts produce the spores, while the others, elongating enormously,
give rise to special cells, called elaters (Fig. 56, _B_).

[Illustration: FIG. 56.--Spore (_A_) and two elaters (_B_) of
_Madotheca_, x 300.]

The ripe spores are nearly globular, contain chlorophyll and drops
of oil, and the outer wall is brown and covered with fine points
(Fig. 56, _A_). The elaters are long-pointed cells, having on the
inner surface of the wall a single or double dark brown spiral band.
These bands are susceptible to changes in moisture, and by their
movements probably assist in scattering the spores after the
sporogonium opens.

Just before the spores are ripe, the stalk of the sporogonium
elongates rapidly, carrying up the capsule, which breaks through the
archegonium wall, and finally splits into four valves, and discharges
the spores.

There are four orders of the liverworts represented in the United
States, three of which differ from the one we have studied in being
flattened plants, without distinct stems and leaves,--at least, the
leaves when present are reduced to little scales upon the lower
surface.

The first order (_Ricciaceae_) are small aquatic forms, or grow on damp
ground or rotten logs. They are not common forms, and not likely to be
encountered by the student. One of the floating species is shown in
figure 57, _A_.

The second order, the horned liverworts (_Anthoceroteae_), are
sometimes to be met with in late summer and autumn, forms growing
mostly on damp ground, and at once recognizable by their long-pointed
sporogonia, which open when ripe by two valves, like a bean pod
(Fig. 57, _B_).

The thi